I think you could consider a horizontal boring machine. Michael G. Rekoff, jnr did an article in an early "fine woodworking" mag with the full plans : it was around issue 35 & reprinted in "Fine woodworking Technics 6".
Giles Gilson from N.Y. did some articles on building your own wooden machines-many types!
if you wanted to go with the drill press you could check out the Oliver No. 194 mortiser for ideas.
Before i purchased 2 Triton routers I used to use a Sissor Jack (as used on old 60's, 70's & 80's vehicles) for a router lift (on a Makita B3600 & a TR12 Hitachi) for many years. You need to make sure the router motor has proper ventilation and I made mine a large U shape from hardwood. It needs to be directly in contact with a solid part of the router casing as well=don't want to crack/damage it. A solid shelf under the Jack will ensure precision with no change in height (I used scraps= 2x 1/2" sheets of ply glued together for the Makita and a 3/4" fibreglassed ply (from a boat) for the TR12).
For one of my drill presses I used form ply with 2 parellel aluminium tracks for locking down odd sized items though for that small machine it could be overkill. The other on has the fence on an arc & no tracks. Both work well.
You may be able to source ideas from Canadian Woodworker, Woodsmith, B. H. & G. Wood, or Fine Woodworking if there is nothing on utube/net.
Go make some shavings/swarf.

Ranald